In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications networks, frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for terminal devices and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network node of the network and at the terminal devices might be required to reach a sufficient link budget.
In a communications network where a terminal device at the user side uses narrow beams for reception (and transmission), at least one of the transmission beams transmitted from a transmission and reception point (TRP) at the network node is assumed to be discovered and monitored by the terminal device. This process of discovering and monitoring at the user side is referred to as beam training. In order to perform beam training the terminal device uses measurements (such as reference signal received power; RSRP) on downlink reference signals (such as channel state information reference signals; CSI-RS). The beam pair for which the highest received reference signal power was obtained is then used as the active beam pair link. In general terms, a beam pair is defined by a transmission beam at the transmitting end (such as at the TRP) and a corresponding reception beam at the receiving end (such as at the terminal device), where the transmission beam and the reception beam are selected from sets of available candidate beams so as to maximize a quality criterion (such as highest reference signal received power) for transmission from the transmitting end to the receiving end.
The CSI-RS for beam training might be transmitted periodically, semi-persistently or a periodically (for example when being event triggered) and they might either be shared between multiple terminal devices or be specific for a certain terminal device, or group of terminal devices.
In order for the terminal device to find a suitable reception beam to receive data and control signalling from the TRP, the TRP transmits CSI-RS in different transmission beams on which the terminal devices perform measurements. FIG. 1 is a signalling diagram of beam training of a terminal device.
S1: The TRP transmits a burst of CSI-RS in a transmission beam with as many occurrences of the CSI-RS in the burst as there are reception beams per antenna array in the terminal device.
S2: The terminal device sweeps through all beams for all its antenna arrays simultaneously. For each beam and antenna array, the terminal performs measurements on the CSI-RS and stores the measurements.
S3: The terminal device evaluates user throughput for all different combinations of reception beams over all its antenna arrays
S4: The terminal device selects a beam setting with one reception beam at each antenna array yielding highest estimated user throughput.
Evaluating the expected user throughput for a certain channel measurement is computationally heavy, which will heat up and increase the power consumption of the terminal device. For example, if the terminal device has an interference rejection combining (IRC) receiver the terminal device has to calculate a matrix inverse, which is considered computationally heavy. If multiple antenna arrays at the terminal device are used, and multiple reception beams at each antenna array, there are many possible beam settings (with one beam from each antenna array per beam setting) to evaluate.
Hence, there is a need for improved beam training.